Many types of industrial facilities, such as for example, steam power plants, require condensation of the steam as integral part of the closed steam cycle. Both wet and dry type cooling towers have been used for condensing purposes. As wet cooled systems consume a considerable amount of cooling water dry cooling systems have gained a growing market share because of their ability to save water resources. In particular, forced draught dry air-cooled condensers consisting of a multitude of fin tube heat exchangers have been known for many years. Contrary to wet cooling arrangements which are characterized by a secondary cooling water loop these systems are so-called “direct” dry systems where steam is directly condensed in the fin tube heat exchangers by air cooling. The fin tube heat exchangers are mounted with the tube center lines arranged in a position inclined to the vertical direction. The bundles are mounted to a support structure which enables cooling air to be conveyed through the fin tube heat exchangers by means of fans. Ambient air in contact with the fin tube heat exchangers condenses the steam inside the fin tubes, which then exits the heat exchanger as condensed sub-cooled liquid. Although being commercially successful over many years a disadvantage of direct dry air-cooled condensers is the power required to operate the fans, as well as fan noise which is undesirable in most situations. Currently 2 types of dry cooling are used, ACC fan assisted, and IDCT natural draft or fan assisted.
Another type of system is the so-called “indirect” dry cooling system. In such a system, a turbine exhaust condenser is provided, where turbine steam is condensed by means of cooling water. The cooling water is conveyed through a water duct by means of a pump to an air-cooled cooling tower which may be of wet or dry type. In the case of dry type the cooling tower consists of a multitude of air-cooled heat exchangers where the heat is rejected to the ambient air by convection. The cooling tower may be operated with fan assistance or in natural draught. The turbine exhaust condenser may for example be a surface or a jet condenser. Because of the presence of a secondary water loop, indirect dry cooling systems are not as thermally effective as direct dry systems. Another disadvantage of natural draught indirect dry cooling systems, however, is the higher investment cost as compared to the forced draught direct air cooled condenser.
Vacuum steam condensers are characterized by ingress of ambient air (inert gas or non-condensables). If not completely withdrawn from the heat exchangers this air will reduce the exchanger efficiency considerably because non-condensables will accumulate and create “air pockets” within the finned tubes. Consequently, effective heat exchange surface and condenser performance will be reduced. Therefore, vacuum condensers are provided with a secondary condenser arranged in reflux mode where the inert gases are extracted from the top exchanger headers of the secondary condenser bundles by special evacuation means. To safeguard that all inert gases are conveyed to these secondary condenser top headers the secondary condenser tube bundles must always be properly supplied by cooling air. Due to local fluctuations of ambient air caused by wind or other reasons natural draught cooled systems may in some instances not be able to maintain permanent secondary condenser cooling while some primary condenser sections are still cooled. This may not only lead to accumulation of inert gases and performance reduction, but also to increase of tube side corrosion as well as the danger of tube side freezing under frost conditions. As long as proper evacuation of the heat exchanger bundles is not guaranteed under all operating conditions the combination of dry condensation and natural draught cooling—although being discussed for some time—poses non-accountable risks to the operator of such equipment.
From the foregoing, although the present direct and indirect systems have proven advantageous, there is a need in the art for other air cooled steam condenser types which can in some cases reduce noise, cost, and/or energy consumption and, at the same time, keep the operational safety as before.